1. Field of the Invention
The present invention relates to optical cables, and more specifically to optical cables comprising a buffer tube, housing the optical fibers, and a sheath around such buffer tube.
A buffer tube loosely housing optical fibers is usually called “loose tube”.
2. Description of the Related Art
These cables are required to have a certain tensile strength both to withstand the stress during cable laying operations and to bear loads applied to the cable itself, for example in case of aerial installation.
To this purpose the cable construction includes one or more strength members. Preferably diametrically opposed strength members are used.
Typically, especially when a limited number of fibers are contemplated for the cable, a central tube construction is used, in which the buffer tube is axially arranged and a sheath coaxially surrounds the buffer tube. Otherwise, a multi tube construction can be used inside the sheath.
In order to maximize the tensile performance of the cable it is important that all the cable constituents work as one and that there is no relative slippage between buffer tube, strength members and cable sheath.
The congruency of the various elements is obtained by embedding the strength members in the cable sheath and selecting the sheath material and the strength members material so that an adequate adhesion is obtained between them.
The thickness of the sheath should be such to cushion the cable elements housed therein, but in some cases other tasks should be considered. For example, when the cable is used in suspension installation, the strength members thereof may be advantageously made in metal to withstand the challenging environmental conditions of this application; however the coexistence of live aerial electric conductors can cause electric voltage arcing across the metallic strength members, thus it is important to provide the cable with a sheath thick enough to hinder this electric flow.
U.S. Pat. No. 5,050,957 relates to an optical fiber cable including a core comprising at least one optical fiber which is enclosed in a relatively rigid tubular member comprised of a plastic material. Disposed about the tubular member is a jacket which is comprised of a plastic material which is substantially less rigid than that of the tubular member. The plastic material of the jacket is characterized by a cut-through resistance which is substantially less than that of the plastic material of the tubular member. Disposed within the jacket and in engagement with the tubular member are two diametrically opposed strength member groups. Each of the strength members is in proximate engagement with the tubular member.
The engagement of the strength members with the tubular member (as well as with the sheath), while providing enhanced strength to the cable, causes the separation of the sheath from the tubular member to become relatively difficult.
The cable sheath should be easily removable. For example, when the cable reaches a customer premise, the buffer tube containing the fiber must be exposed and the optical fibers accessed to be connected to a user's apparatus or to corresponding fibers of other cables.
U.S. Pat. No. 4,456,331 illustrates a communications cable comprising a core, a plurality of channels each of which is dimensioned to contain a layer of optical fibres running along the outer peripheral surface of the core and an external sheath covering the core, the sheath being openable at locations positioned over at least some and preferably all of the channels whereby external access to the fibres in the channel can be obtained. Preferably a removable strip of sheath defined by zones of weakness extends over each channel.
In this cable the fibres are housed in channels made in the sheath itself and are not protected by an independent tubular housing.
U.S. Pat. No. 5,067,830 addresses the problems of cable access in mid-span and describes a tube holding an optical waveguide, the tube having a first lengthwise indentation in its outer surface and a second lengthwise indentation in its inner surface adjacent to the first lengthwise indentation. The cable tube may also have a third lengthwise indentation in its outer surface located 180° from the first lengthwise indentation, and a fourth lengthwise indentation in its inner surface located 180° from the second lengthwise indentation. With this arrangement, a tube slitting tool may be used with less danger to optical waveguides within the tube.
In this cable the fibres are arranged in ribbons rigidly housed in a tube.
A common way for removing the cable sheath from around the buffer tube provides the step of making two longitudinal cuts along the cable sheath. The cuts must be provided between the strength members to ensure the cutter blade to go through to the outside of the buffer tube. For helping the identification of the cut position, grooves can be provided on cable sheath surface, as shown for example in FR 2633402.
However, it is difficult to pull the cutter along the cable, both for the pulling force required cutting the whole thickness of the sheath and because the cut depth has to be carefully controlled to slice the sheath substantially completely, but without damaging the underlying buffer tube.